This invention relates generally to the field of implant-patterning oxide superconductive material, and more specifically pertains to a method for manufacturing implant-patterned superconducting devices by ion implantation of superconducting films through a passivation layer.
The methods for fabrication of integrated circuits based on high critical temperature oxide superconductive (HTS) films have evolved rapidly over the past several years. But, further improvements of fabrication technologies of HTS electronic devices are still necessary due to difficulties in obtaining epitaxial insulating layers and providing reliable patterning processes, especially for multi-layered devices.
Patterning can be accomplished by any appropriate process, such as conventional photolithography and dry etching. For fabrication of YBaCuO (YBCO) and similar HTS integrated circuits, the current technology involves patterning a YBCO film with photoresist followed by ion-milling areas of the film not covered by photoresist, to remove material and form different structures, such as YBCO lines. Complicated three-dimensional architectures are constructed by removing the photoresist, depositing additional films, additional patterning with photoresist, ion-milling, etc.
Other suitable patterning methods for production of implanted pattern regions include introduction of impurities into a superconductor by planar diffusion techniques and ion implantation processes. A superconductor may be ion implanted through an overlaying mask or by scanning the ion beam in a predetermined pattern. As contrasted with the diffusion process, in the ion implantation process, the number of implanted ions is controlled by the external system parameters, such as the ion source type and accelerating energy. The depth of penetration is a function of the kinetic energy of the impurity ions, the crystalline structure, and, the mass of the recipient atoms. Further, the implantation may be carried out at low temperatures at which the diffusion process cannot be performed. The ion implantation process can be used in combination with previously diffused device structures without affecting these prior structures.
Ion implantation has recently been used with HTS materials, such as YBCO materials, to obtain implant-patterned YBCO films by introduction of a reactive impurity, such as Si, to remove oxygen (O) from an oxide superconductor. Introduction of Si ions into a YBCO HTS material breaks down the Cu--O chemical bonds with the Si itself becoming oxidized to form an insulating oxide compound. The oxide superconductors include La--Sr--Cu--O, Ca--Sr--Cu--O, Y--Ba--Cu--O, Bi--Sr--Ca--Cu--O, Th--Ba--Ca--Cu--O, Hg--Ba--Ca--Cu--O, Bi--K--Ba--O, Nd--Ba--Cu--O, etc. The reactive impurity may be any one of the elements from the group of elements which are more reactive with oxygen than the element in the oxide superconductor (e.g. Cu, Ba). Elements such as Si, Th, Al, Mg, Sr, Ni, B, Ce, Ge, Fe, Zr, or Nb, and compounds such as Si.sub.3 Ni.sub.4, SiF.sub.2 or SiF.sub.3 are suitable reactive impurities.
Studies reveal that chemical reactions between implanted Si ions and a YBCO material, and the consequential formation of Si oxide, inhibit the superconducting characteristics of HTS film causing the affected film portion to acquire an electrical insulating characteristic in place of its former superconducting characteristic. Accordingly, silicon can be used to pattern HTS YBCO films by locally inhibiting superconductivity in selected portions of the film.
However, consistent growth of good quality superconducting films on conventionally implanted YBCO layers has not been successful. One challenge encountered in developing a successful ion implantation patterning process for a multi-layer YBCO device is the inability to pattern micron-level superconducting lines which are electrically isolated from each other while, at the same time, maintaining the isolation and pattern definition after exposure to the high temperature anneals associated with additional depositions. In addition, with conventional technology it is difficult to efficiently grow epitaxial superconducting YBCO films on top of ion implant-patterned YBCO films because the crystalline template of the implanted YBCO film surface, required for additional growth, is often destroyed during the ion implantation process and newly-grown layers have large amounts of crystalline damage. As each ion is accelerated into the YBCO film and careens through the crystal lattice, it can create 100's to 1000's of vacancy and interstitial defects. In addition to this common form of implant damage, YBCO has a propensity to develop strain domains which are characterized by twinning. Further, ion implantation of YBCO films can cause the strain domains to migrate within the plane of the film which, in extreme cases, can lead to microcracking. Also, YBCO materials readily form undesirable secondary phases with the implanted ion species which can lead to surface buckling.
Therefore, there is a need for an efficient method for fabrication of superconducting patterns by ion implantation of a superconducting material at high production yields. The method should allow efficient fabrication of devices with multiple layers of superconductive materials without degradation of the superconductive material's crystalline structure.